Method of forming solder bumps

ABSTRACT

A method of forming solder bumps includes preparing a substrate having a surface on which a plurality of electrode pads are formed, forming a resist layer on the substrate, the resist layer having a plurality of openings, each of the openings being aligned with a corresponding electrode pad of the plurality of electrode pads, forming a conductive pillar in each of the openings of the resist layer, forming conductive layers to cover at least side walls of the resist layer in the openings to block gas emanating from the resist layer, filling molten solder in each of the openings in which the conductive layers has been formed and removing the resist layer.

BACKGROUND OF THE INVENTION Technical Field

The present invention generally relates to a method of forming solderbumps, and more specifically, to a method of forming solder bumps usinginjection of molten solder.

Description of the Related Art

While performance and function of electronics devices improve highly,flip chip packaging is broadly applied from the consumer productrepresented by a smart phone, tablet PC, etc. to the supercomputer.Furthermore, it is predicted that the demand of flip chip packagingincreases sharply by the appearance of the 2.5 or 3-dimensional (2.5D or3D) stacked device of the semiconductor chip.

In the 2.5D or 3D package, connecting terminals pitch and bump size isdramatically fine. In that case, there is a problem of failure due tostress applied to the junction or failure due to electro migration (EM)caused by the increase of current density. To solve this problem, thesolder bumps using Cu pillars corresponding to the miniaturization ofthe connecting terminals pitch and bump size is mainly used.

As a method of forming solder on Cu pillars, Injection Molded Solder(IMS) method has been developed. In IMS method, molten solder isdirectly injected into openings of a resist mask at higher temperaturethan solder melting point. In the process, the degree of vacuum in theopening of the resist mask is reduced and contamination of the electrodesurface is generated by gas which is generated from the resist mask. Asa result, the solder is not filled sufficiently in the opening or thesolder does not wet and spread to the electrode pads.

One of the solutions is to develop a resist material which has lowoutgas at high temperature, but it is difficult to eliminate outgascompletely because the resist should be strippable after IMS. As an IMSprocess, a solution is to increase the injection pressure. However, byincreasing the injection pressure, there is a possibility that solderleakage occurs. To suppress solder leakage, it is necessary to increasethe pressing pressure of the IMS head, in which case, there is apossibility of deforming the resist material.

Thus, in the conventional method of forming fine solder bumps using IMS,it is not easy to make uniform and low defect solder bumps. Therefore,there is a need of method to form fine solder bumps with high uniformand low defects.

SUMMARY

The present invention provides a method of forming solder bumps on thesubstrate using injection of molten solder such as IMS. In the method,conductive layers which cover at least side walls of the resist layer inthe openings is formed before injecting solder to block gas emanatingfrom the resist layer. The conductive layers can include at least ametal with solder wettability and/or higher melting point than materialsof the molten solder. In the process, the substrate temperature can becontrolled to a temperature lower than the melting point of theconductive layers to prevent the deposited layers from melting andaggregating.

The conductive layers suppress the invasion of outgas of the resistopening. Suppressing the invasion of outgas of the resist opening by theconductive layers maintains the degree of vacuum in the openings andinhibits the contamination of the electrode surfaces on the substrate.Since the conductive layers with solder wettability on the surfaces inthe openings are present, they make it easier to solder injection intothe openings at a lower pressure in IMS process. Accordingly, incomparison with no metal layer, high solder filling rate is achieved ata low solder injection pressure, and the risk of solder leakage isreduced. Since the conductive layers dissolve into the molten solder inIMS process, it is possible to expose the resist surface and allowsubsequent resist removal (etching).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a blow/flow diagram showing a system/method of forming solderbumps in accordance with one embodiment of the present invention;

FIG. 2 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 3 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 4 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 5 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 6 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 7 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 8 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 9 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 10 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 11 is a cross-sectional diagram showing a solder bump structure inaccordance with one embodiment of the present invention;

FIG. 12 is a block/flow diagram showing a system/method of formingsolder bumps in accordance with an embodiment of the present invention;

FIG. 13 is a cross-sectional diagram showing a solder bump structure inaccordance with an embodiment of the present invention;

FIG. 14 is a cross-sectional diagram showing a solder bump structure inaccordance with an embodiment of the present invention;

FIG. 15 is a cross-sectional diagram showing a solder bump structure inaccordance with an embodiment of the present invention;

FIG. 16 is a cross-sectional diagram showing a solder bump structure inaccordance with an embodiment of the present invention; and

FIG. 17 is a cross-sectional diagram showing a solder bump structure inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The following description is an explanation of embodiments of thepresent invention with reference to the drawings. FIG. 1 is a block/flowdiagram showing a system/method of forming solder bumps in accordancewith an embodiment of the present invention. Each of FIG. 2 to FIG. 11shows cross-sectional diagram of a solder bump structure at each step inthe flowchart of FIG. 1. FIG. 12 is a block/flow diagram showing asystem/method of forming solder bumps in accordance with an embodimentof the present invention. FIG. 13 to FIG. 17 show cross-sectionaldiagrams of a solder bump structure at steps selected from the flowchartof FIG. 12. Each of the cross-sectional diagrams shows a part of asubstrate. The following description is an explanation of theembodiments of the present invention with reference to FIG. 1 to FIG.17.

Embodiment 1

Referring FIG. 2, in step S1 of FIG. 1, a substrate 10 is preparedfirst. The substrate 10 has a surface on which electrode pads 14 areformed between patterned insulating layers 12. The electrode pads 14 caninclude metal, such as aluminum (Al), for example. The insulating layers12 can include silicon oxide (SiO₂), for example. The substrate 10 caninclude semiconductor wafer, such as a Si wafer, a semiconductorchip/die, or a circuit board. The material of the semiconductor wafer orchip is not limited to specific types. The substrate 10 can include aplurality of wiring layers (including circuits, devices such as atransistor) and insulating layers. The wiring layers can electricallyconnect to the electrode pads 14. The substrate 10 may include alamination (e.g., stack) of a plurality of semiconductor substrates.

Referring to FIG. 3, in step S2 of FIG. 1, at least one seed layer 15,16 is formed on the substrate 10. The at least one seed layer 15, 16 isused when using electro plating to form conductive pillars 22 in thelater step S4. If not using electro plating to form the conductivepillars 22, it is not necessary to form the seed layer 15, 16. The seedlayers 15, 16 may include a plurality of conductive layers. Theconductive layers may include a first conductive layer 15, and a secondconductive layer 16 on the first conductive layer 15, as shown in FIG.3. The first conductive layer 15 may include Ti or TiW, for example. Thesecond conductive layer 16 may include Cu, for example. The seed layers15, 16 may be formed by a conventional thin-film formation techniquesuch as sputtering or plasma CVD, for example.

Referring FIG. 4, in step S3 of FIG. 1, a resist layer 17 is formed onthe substrate 10. A resist material is applied on the substrate 10 usingspin coating for example, and the resist material is cured at apredetermined temperature to form the resist layer 17. The resistmaterial may include light (UV)-curable resin (photopolymer) orthermosetting resin (polymer). The resist layer 17 may include one ofnegative tone type resist or positive tone type resist.

Next, the resist layer 17 is exposed using light induced through a photomask (not shown) in FIG. 4. For example, if the resist layer 17 isnegative tone type, the exposure is performed to expose only the resistregion over the insulating layers 12 without exposing the resist regionover the electrode pads 14. Next, the exposed resist layer 17 isdeveloped and patterned to form resist layer 18 with openings 20 overthe electrode pads 14, as shown in FIG. 5. Each of the openings 20exposes the surface of the top seed layer 16 over the electrode pads 14on the substrate 10. When the surface of the seed layer 16 is easilyoxidized, it is necessary to remove the oxide layer formed on thesurface before the next step. The oxide layer can be removed by etchingusing an acidic solution, for example.

Next, in step S4 of FIG. 1, conductive pillars 22 are formed in theopenings 20 to connect to the electrode pads 14 via the seed layers 15,16, as shown in FIG. 6. The conductive pillars 22 may be formed byelectroplating conductive material in the openings 20 of the resistlayer 18 to leave space (e.g., a cavity) 23 with a predetermined depthon the conductive pillar 22 in the openings 20. The conductive materialmay include Cu, for example. The conductive pillar 22 can also be formedbefore step S3. The conductive pillar 22 can be formed by otherconventional metal formation technique such as electro-less plating,sputtering or plasma CVD, for example.

Next, in step S5 of FIG. 1, a conductive layer 24 is formed on thesubstrate as shown in FIG. 7. In FIG. 7, the conductive layer 24 is acontinuous layer and covers the surfaces of the conductive pillars 22and surfaces of the resist layer 18. The conductive layer 24 may beformed by conventional metal formation technique, such as vacuumdeposition, sputtering, electroless plating or plasma CVD, for example.The conductive layer 24 is used to block gas emanating from the resistlayer 18 in the next solder injection process. The conductive layer 24may include at least a metal with solder wettability and/or highermelting point than materials of the molten solder which is used in thenext solder injection process. The conductive layer 24 may include atleast one of metals selected from the group comprising Sn, Au, Ag, Cu,Pd, Pt and In.

Next, in step S6 of FIG. 1, molten solder 28 is filled in the space 23on the conductive pillar 22 as shown in FIG. 8 and FIG. 9. The moltensolder 28 is injected in the space 23 on the conductive layer 24 usingInjection Molded Solder (IMS) process, for example, as shown in FIG. 8.In the IMS process, the space 23 is first vacuumed and the molten solder28 is injected in the space 23 which has been vacuumed under thepredetermined temperature and pressure by the IMS head 26. Thepredetermined temperature is set as the predetermined substratetemperature and controlled to a temperature lower than the melting pointof the conductive layer 24 in the IMS process. The peak of the substratetemperature, in other words, the substrate temperature when the IMS head26 passes on the opening 23 which has been vacuumed is controlled abovethe melting point of the solder.

The predetermined pressure for the injection is determined according tothe materials of the tip of the IMS head 26. For example, the tipmaterial of the IMS head 26 includes silicone sponge and low frictionsheet, the pressure is set to 0.02-0.2 MPa as the differential pressurebetween the atmospheric pressure, depending on size of openings. The IMShead 26 repeats evacuation and solder injection while moving in thehorizontal direction in FIG. 8.

In the injection of molten solder, the conductive layer 24 blocks gasemanating from the resist layer 18 and maintains the degree of vacuum inthe space 23, inhibiting the contamination of the conductive pillars 22on the substrate 10. The conductive layer 24 is dissolved in the moltensolder 28 with high temperature to be injected. As a result, theconductive layer 24 loses its form of layers after the solder injectionas shown in FIG. 9. Since the conductive layer 24 dissolves into themolten solder 28 in IMS process, it is possible to expose the resistsurface and allow subsequent resist removal (etching) without additionalprocess of etching the conductive layer 24.

Since the conductive layer 24 with solder wettability on the surfaces inthe space 23 are present, they make it easier to solder injection intothe spaces 23 at a lower pressure in IMS process. Accordingly, incomparison with no conductive layer, high solder filling rate isachieved at the low solder injection pressure, and the risk of solderleakage is reduced. The molten solder 28 may include a Pb-free soldermetal containing at least one metal selected from the group consistingof elemental Sn, Ag, Au, Cu, Ni, Bi, In, Zn, Co, Ge, Fe and Ti, andcontaining Sn or In as a main component. The injected molten solder 28is cooled and has a shape with convex top surface, as shown in FIG. 9.

Next, in step S7 of FIG. 1, the resist layer 18 is removed usingconventional etching process and the surface of the seed layer 16 isexposed as shown in FIG. 10. Finally, in step S8 of FIG. 1, the seedlayers 15, 16 on the insulating layer 12 is removed using conventionaletching method and the solder bumps 29 is obtained, as shown in FIG. 11.Each of the solder bumps 29 includes the solder 28 on the conductivepillar 22 connected to the electrode pad 14 via the seed layers 15, 16on the substrate 10.

Embodiment 2

Referring FIG. 12 to FIG. 17, an embodiment of the present invention isdescribed. Steps S10 to S40, and S70 to S90 in FIG. 12 are the same assteps S1 to S4, and S6 to S8 in FIG. 1 respectively, as described above.Steps S50 and S60 are added in FIG. 12. In step S50 of FIG. 12,conductive paste is filled in the space 23 on the conductive pillar 22in FIG. 6. FIG. 13 shows a cross-sectional diagram of a solder bumpstructure after step S50, which performs filling the conductive paste 30in the space 23. The conductive paste 30 can be filled in the space 23of the resist layer 18 using screen-printing technique or injectingtechnique, for example. The conductive paste 30 can include metalnanoparticles in a solvent. The metal nanoparticles may include at leasta metal with solder wettability and higher melting point than materialsof the molten solder 28 which is used in the later solder injectionprocess. The metal nanoparticles may include at least one of metalsselected from the group comprising Sn, Au, Ag, Cu, Pd, Pt, Ni and In,for example. The viscosity of the conductive paste 30 and the particlefraction in the conductive paste 30 may be determined in considerationof paste shrinkage, in other words, the thickness of a conductive layerto be obtained by the next sintering process.

Next, in step S60 of FIG. 12, the conductive paste 30 in the spaces 23is sintered to form a thin conductive layer 32. The sintering of theconductive paste 30 in the opening is performed to heat the conductivepaste 30 at 150 to 250 degrees Celsius for 0.5 to 1.5 hours in anatmosphere of nitrogen gas or formic acid to prevent oxidation of themetal surface after the sintering. If the sintering is performed in air,it is necessary to remove the oxide layer on the metal surface. In thesintering process, the conductive paste 30 shrinks so that the thinconductive layers 32 are formed as shown in FIG. 14. The thin conductivelayers 32 cover the inner side walls of the resist layer 18 and thesurface of the conductive pillars 22, as shown in FIG. 14. The thinconductive layers 32 are only formed in the space 23 (34) withoutcovering the top surfaces of the resist layer 18. The thin conductivelayers 32 are used to block gas emanating from the resist layer 18 inthe next solder injection process, as in the case of the conductivelayers 24 in FIG. 8.

As solder filling is performed in the next step without additionalconductive paste coating, the volume shrinkage of the conductive paste22 after sintering is optimized. The volume shrinkage of the conductivepaste 22 is dependent on the design value of the bump diameter/height,and for example is preferably at least 50% or more. In other words, thethickness of the conductive layers 32 after the sintering is set as thethickness required to block outgas from the resist layers 18 in the nextsoldering process. The space 34 is formed on the conductive layer 32which leads to the upper end of the opening 20. The conductive layer 32has a cone-shaped surface 36, as shown in FIG. 14. The cross-section ofthe conductive layer 32 has a conformal shape.

Next, in step S70 of FIG. 12, molten solder 38 is filled in the space 34on the conductive pillar 22 as shown in FIG. 15. The injected moltensolder 38 is cooled and has a shape with convex top surface as shown inFIG. 15. The molten solder 38 is in contact with and extends an entirelength of the cone-shaped surface of the conductive layer 32. The moltensolder 38 is injected in the space 34 on the conductive layer 32 usingIMS process, for example, as in the case of the molten solder 28 in FIG.8. As described above referring FIGS. 8 and 9, in the IMS process, thespace 34 is first vacuumed and the molten solder 38 is injected in thespace 34 which has been vacuumed under the predetermined temperature bythe IMS head 26 in FIG. 8. The IMS head 26 repeats evacuation and solderinjection while moving in the horizontal direction in FIG. 8. In theinjection of molten solder 38, the conductive layers 32 block gasemanating from the resist layer 18 and maintain the degree of vacuum inthe space 34, inhibiting the contamination of the conductive pillars 22on the substrate 10.

Each conductive layer 32 is dissolved in the molten solder 38 with hightemperature to be injected. As a result, at least part of eachconductive layer 32 loses its form as a layer after the solder injectionas shown in FIG. 15. Since the top surfaces of the resist layers 18remain exposed in the soldering process, it is possible to allowsubsequent resist removal (etching) without additional process ofetching the conductive layers 32.

Next, in step S80 of FIG. 12, the resist layer 18 is removed using aconventional etching process and the surface of the seed layer 16 isexposed as shown in FIG. 16. In step S90 of FIG. 12, the seed layers 15,16 on the insulating layer 12 is removed using conventional etchingmethod and the solder bumps 34 of an embodiment of the present inventionis obtained, as shown in FIG. 17. Each of the solder bumps 34 includesthe solder 38 on the conductive pillar 22 connected to the electrode pad14 via the seed layers 15, 16 on the substrate 10.

Embodiment 3

Referring FIG. 12 to FIG. 15, an embodiment of the present invention isdescribed. In an embodiment, the filling of the conductive paste 30 instep S50 of FIG. 12 is performed using IMS process which is used in stepS70 of filling of molten solder 38 instead of screen printing. In stepS50, the conductive paste 30 is injected in place of molten solder 38under predetermined pressure by IMS process. By using the IMS in StepS50, it is possible to perform steps S50 to S70 under one IMS process.That is, sintering of step S60 can be performed in the IMS process. As aresult, it is possible to achieve a further shortening of the productiontime and simplification of the manufacturing process to form the solderbumps 34.

Embodiment 4

Referring FIG. 1 and FIG. 7, an embodiment of the present invention isdescribed. In an embodiment, the conductive layer 24 is formed as two ormore layers on the on the substrate 10 to cover the surfaces of theconductive pillars 22 and surfaces of the resist layer 18. In this case,it is possible to form layers made of different conductive materials.For example, a first conductive layer having melting point higher thanthat of the solder 28 is formed and a second conductive layer havinggood solder wettability is formed on the first conductive layer. In theIMS process of step S6 in FIG. 1, FIG. 8, the predetermined substratetemperature is set to a temperature lower than the melting point of thefirst conductive layer.

Embodiment 5

Referring FIG. 1 and FIG. 7, an embodiment of the present invention isdescribed. In an embodiment, the conductive layer 24 is formed byconductive metals having the same composition ratio as the compositionratio of metals of the molten solder 28 in order to maintain thecomposition ratio of the solder after the IMS process. For example, ifthe composition ratio of the molten solder 28 used by IMS process is Sn(96.5 wt %)/Ag (3.0 wt %)/Cu (0.5 wt %), the conductive layer 24 caninclude the same composition ratio of Sn (96.5 wt %)/Ag (3.0 wt %)/Cu(0.5 wt %).

The embodiments of the present invention have been described withreference to the accompanying drawings. However, the present inventionis not limited to the embodiment. The present invention can be carriedout in forms to which various improvements, corrections, andmodifications are added based on the knowledge of those skilled in theart without departing from the purpose of the present invention.

What is claimed is:
 1. A method of forming solder bumps, the methodincludes: preparing a substrate having a surface on which a plurality ofelectrode pads are formed; forming a resist layer on the substrate, theresist layer having a plurality of openings, each of the openings beingaligned with a corresponding electrode pad of the plurality of electrodepads; forming a conductive pillar in each of the openings of the resistlayer; forming conductive layers, including two or more metal layers, tocover at least side walls of the resist layer in the openings, theconductive layers being configured to block gases emanating from theresist layer as a result of outgassing; filling molten solder in avacuum formed in each of the openings in which the conductive layers hasbeen formed, wherein the conductive layers have a higher melting pointthan materials of the molten solder; and removing the resist layer. 2.The method according to claim 1, wherein filling the molten solder inthe vacuum formed in each of the openings includes injecting the moltensolder in each of the openings under predetermined injecting pressureand predetermined substrate temperature.
 3. The method according toclaim 2, wherein injecting the molten solder in each of the openings isperformed using an Injection Molded Solder (IMS) method.
 4. The methodaccording to claim 2, wherein the predetermined substrate temperature islower than melting points of metals constituting the conductive layers.5. The method according to claim 1, wherein the conductive layersinclude at least a metal with solder wettability/higher melting pointthan materials of the molten solder.
 6. The method according to claim 1,wherein the conductive layers include at least one metal selected fromthe group comprising Sn, Au, Ag, Cu, Pd, Pt, and In.
 7. The methodaccording to claim 1, wherein the conductive layers include metalshaving a same composition ratio as a composition ratio of metals of themolten solder.
 8. The method according to claim 1, further comprising:forming at least one seed layer on the substrate before forming theresist layer; and removing the at least one seed layer after removingthe resist layer.
 9. The method according to claim 1, wherein formingthe conductive pillar in each of the openings of the resist layerincludes electroplating conductive material in the opening of the resistlayer.
 10. The method according to claim 1, wherein the electrode padsare formed between patterned insulating layers.
 11. A method of formingsolder bumps, the method includes: preparing a substrate having asurface on which a plurality of electrode pads are formed; forming aresist layer on the substrate, the resist layer having a plurality ofopenings, each of the openings being aligned with a correspondingelectrode pad of the plurality of electrode pads; forming a conductivepillar in each of the openings of the resist layer; forming conductivelayers, including forming the conductive layers using vapor deposition,sputtering, or electroless plating, to cover at least side walls of theresist layer in the openings, the conductive layers being configured toblock gases emanating from the resist layer as a result of outgassing;filling molten solder in a vacuum formed in each of the openings inwhich the conductive layers has been formed; and removing the resistlayer.
 12. The method according to claim 11, wherein filling the moltensolder in the vacuum formed in each of the openings includes injectingthe molten solder in each of the openings under predetermined injectingpressure and predetermined substrate temperature.
 13. The methodaccording to claim 12, wherein injecting the molten solder in each ofthe openings is performed using an Injection Molded Solder (IMS) method.14. The method according to claim 12, wherein the predeterminedsubstrate temperature is lower than melting points of metalsconstituting the conductive layers.
 15. The method according to claim11, wherein the conductive layers include at least a metal with solderwettability/higher melting point than materials of the molten solder.16. The method according to claim 11, wherein the conductive layersinclude at least one metal selected from the group comprising Sn, Au,Ag, Cu, Pd, Pt, and In.
 17. The method according to claim 11, whereinthe conductive layers include metals having a same composition ratio asa composition ratio of metals of the molten solder.
 18. The methodaccording to claim 11, wherein the conductive layers include two or moremetal layers.
 19. The method according to claim 11, further comprising:forming at least one seed layer on the substrate before forming theresist layer; and removing the at least one seed layer after removingthe resist layer.
 20. The method according to claim 19, wherein formingthe conductive pillar in each of the openings of the resist layerincludes electroplating conductive material in the opening of the resistlayer.